Sort Method: external merge

Hi,

I have several queries in *single* transaction and I want to figure
out reasonable work_mem value. Here is the excerpt from "explain plan"
-- each query has two sorts:
1)               Sort Method:  quicksort  Memory: 6 324kB
                           Sort Method:  quicksort  Memory: 1 932 134kB

2)               Sort Method:  quicksort  Memory: 28 806kB
                           Sort Method:  quicksort  Memory: 977 183kB

3)               Sort Method:  quicksort  Memory: 103 397kB
                           Sort Method:  external merge  Disk: 3 105 728kB

4)               Sort Method:  quicksort  Memory: 12 760kB
                           Sort Method:  quicksort  Memory: 3 704 460kB

5)               Sort Method:  quicksort  Memory: 84 862kB
                           Sort Method:  external merge  Disk: 3 593 120kB

6)               Sort Method:  quicksort  Memory: 4 828kB
                           Sort Method:  quicksort  Memory: 112 472kB

7)               Sort Method:  quicksort  Memory: 1 490kB
                           Sort Method:  quicksort  Memory: 81 066kB

8)               Sort Method:  quicksort  Memory: 78174kB
                           Sort Method:  quicksort  Memory: 2 579 739kB

9)               Sort Method:  quicksort  Memory: 101 717kB
                           Sort Method:  quicksort  Memory: 2 913 709kB

work_mem is set to 4 000 000 kb and I do not understand why few
queries (3 and 5) used disk and the rest fit were able to data into
memory. Why disk was used and subsequent query was able to sort bigger
data set in the memory (see 3 and 4)? The box has 90GB RAM and no
other queries run during that time.

PG version is: PostgreSQL 8.4.7 on x86_64-redhat-linux-gnu, compiled
by GCC gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-50), 64-bit

Thanks,
--
Ondrej Ivanic
(ondrej.ivanic@gmail.com)